Method and apparatus for error control in 3D video transmissoin

ABSTRACT

Methods and apparatuses for error control in 3D video transmission over wireless network are described. Cooperative Automatic Repeat re Quest(ARQ) is disclosed, which is based on the selective repeat ARQ with consideration of two factors: one is the interdependent relationship between the multiple components of the 3D video, e.g. the 2D video and its depth information in the 2D-plus-Depth format; and the other is time constraints of video frames/packets for continuous video playback. The disclosed cooperative ARQ allows the sender to control ARQ strength adaptively on a per-frame/per-packet basis.

This application claims the benefit, under 35 U.S.C. §365 ofInternational Application PCT/CN2012/086522 filed Dec. 13, 2012 whichwas published in accordance with PCT Article 21(2) on Jun. 19, 2014 inEnglish.

TECHNICAL FIELD

The present invention generally relates to error control for datatransmission. More particularly, it relates to error control for 3Dvideo transmission.

BACKGROUND OF THE INVENTION

Streaming 3D video over a wireless network is a very challenging taskdue to the highly variable nature of wireless networks. Packets may bedropped during transmission or may only reach the destination after asignificant delay. One of the crucial limitations of wireless networksis related to the high Bit Error Rate (BER) on the radio link. In orderto counteract high BERs, most of the wireless network technologiesemploy Automatic Repeat reQuest (ARQ).

ARQ (Automatic Repeat reQuest), also known as Automatic Repeat Query, isan error-control method for data transmission. In conventional ARQschemes, packet errors are examined at the receiver by an errordetection code, usually cyclic redundancy checks (CRC). If the packet isreceived successfully, the receiver sends an acknowledgement (ACK) ofthe successful transmission to the receiver. If the receiver detects anerror in the packet, it sends a negative acknowledgement (NACK) requestfor re-transmission of that packet.

If the sender does not receive an ACK before a timeout, which is aperiod of allowed lapse time before an acknowledgement is to bereceived, or if it receives a NACK, it usually re-transmits the packetuntil an ACK is received or a predefined number of re-transmissions arereached.

The basic ARQ schemes include stop-and-wait ARQ, Go-Back-N ARQ, andselective repeat ARQ.

Stop-and-wait ARQ is inefficient compared with other ARQs, because thesender does not send any further packets until it receives an ACKsignal. To solve this problem, one can pick a window, and send the setof packets falling inside the window one by one before an earlier packetis acknowledged and use one ACK signal for each packet. This is thebasic idea of go-back-N ARQ and the selective repeat ARQ.

Go-back-N ARQ uses a window mechanism where the sender can send packetsthat fall within a window. The window advances as acknowledgements forearlier packets are received. The receiver receives packets in order andcannot accept packets out of sequence. Go-back-N ARQ uses networkconnection more efficiently than stop-and-wait ARQ. Instead of waitingfor an acknowledgement for every packet, the connection is utilized asthe set of packets within the window are sent. However, this method alsoresults in sending packets multiple times—if any packet is lost ordamaged, or the ACK signal acknowledging them is lost or damaged, thatpacket and all following packets falling in the window will be re-sent,even if they were received without error. To avoid this, selectiverepeat ARQ can be used.

Unlike go-back-N ARQ, the receiving process of selective repeat ARQ willcontinue to accept and acknowledge packets sent after an initial error.The receiver acknowledges each successfully received packet bytransmitting an ACK bearing the sequence number of the packet beingacknowledged. If an ACK is not received for a packet before theexpiration of the timeout, the packet is retransmitted. Once a packethas been retransmitted the transmitter resumes transmission of packetsfrom where it left off.

Various schemes exist that mitigate the effects of errors duringtransmission of 2D video data. However, the transmission of 3D videodata has not been investigated in the art with consideration of 3D videocharacteristics.

SUMMARY OF THE INVENTION

This invention is directed to methods and apparatuses for error controlin 3D video transmission.

According to an aspect of the present invention, there is provided amethod for controlling errors during data transmission, wherein the datato be transmitted comprise two interrelated components. The methodcomprises transmitting a first packet of a first component and a secondpacket of a second component of the data separately; determining are-transmission probability for the second packet based on atransmission status of the first packet; and re-transmitting the secondpacket with the re-transmission probability for the second packet.

According to another aspect of the present invention, there is providedan apparatus for controlling errors during data transmission, whereinthe data to be transmitted comprise two interrelated components. Theapparatus comprises a normal transmission unit for transmitting a firstpacket of a first component and a second packet of a second component ofthe data separately; a re-transmission determination unit for generatinga re-transmission request for the second packet based on a transmissionstatus of the first packet; and a re-transmission unit forre-transmitting the second packet upon receiving the re-transmissionrequest.

BRIEF DESCRIPTION OF THE DRAWINGS

The above features of the present invention will become more apparent bydescribing in detail exemplary embodiments thereof with reference to theattached drawings in which:

FIG. 1 shows exemplary cooperative ARQ modules according to anembodiment of the present invention: (a) 2D ARQ module; and (b) DepthARQ module.

FIG. 2 shows an exemplary process of controlling errors in the 3D videotransmission according to one embodiment of the present invention.

FIG. 3 illustrates relationships among various timeparameters/quantities in one embodiment of the present invention.

DETAILED DESCRIPTION

This invention provides a method for error control in 3D videotransmission over wireless networks. 3D video can take various formatssuch as 2D-plus-Depth, stereo format with left and right views,multi-view, 2D-plus-Delta format etc. The common feature of these 3Dvideo formats is that there are multiple components/files in a 3D videofile, and there is a one to one correspondence between these multiplecomponents/files. For example, the 2D-plus-Depth format has a 2D videocomponent/file and a depth map component/file, and one frame of the 2Dvideo corresponds to one frame of the depth map. The stereo format has aleft view video component/file and a right view video component/file,and one frame in the left view has a corresponding frame in the rightview.

In the following, 2D-plus-Depth is used as an example 3D video format.The same principles apply to other 3D video formats. 2D-plus-Depth isone popular format for representing 3D video, wherein a 2D video issupplemented with a depth map. The depth map represents the per pixeldistance from the camera and can be used to render 3D video at theuser's terminal. This format is attractive since the inclusion of depthenables a display-independent solution for 3D. It allows adjustment ofdepth perception in stereo displays according to viewing characteristicssuch as display size and viewing distance. Moreover, it is backwardcompatible with legacy 2D set-top boxes and is display formatindependent. Transmitting 2D video and depth information in twodifferent logical channels provides flexibility to viewers. For example,a viewer who has only a 2D decoder only needs to receive 2D video andviews the 2D video normally. He/she can choose whether to receive theassociated depth information or not, which cannot be achieved if 2Dvideo and the depth map are multiplexed. Here logical channels refer tothe channels where data can be received separately, including throughnon-physical methods, e.g. through different IP multicast addresses.

This invention provides an error control method for3D videotransmission, called “cooperative ARQ”. It is based on the selectiverepeat ARQ with consideration of two factors. One is theinterdependent/interrelated relationship between the multiple componentsof the 3D video, i.e. the 2D video and its depth information in the2D-plus-Depth format. The other is time constraints for continuous videoplayback. It allows the sender (or a mobile terminal) to control ARQstrength adaptively on a per-frame or per-packet basis. It guaranteescontinuous playback of the 3D video and efficient use of networkconnections.

During the transmission of the 3D video, both 2D video data and depthdata are put into packets with one packet of 2D video data correspondingto one packet of depth data. Depending on the applications, one packetmay have a size smaller or larger than one frame of the 2D video and thedepth map.

FIG. 1 illustrates cooperative ARQ modules according to one embodimentof the present invention. It comprises a 2D ARQ module 100 and a depthARQ module 110. Each of the two ARQ modules comprises a normaltransmission unit (101 and 111) for transmitting packets of thecorresponding data, i.e. 2D video (101) or the depth map (111), anacknowledgement receiving unit (102 and 112) for receiving the ACK orNACK signals for the sent packets, a re-transmission determination unit(103 and 113)for generating a re-transmission request for sent packetsbased on a transmission status of sent packets and a time constraint,and a re-transmission unit (104 and 114) for re-transmitting packetsupon receiving a re-transmission request.

In a different embodiment, the corresponding units for the two ARQmodules can be combined. For example, one transmission unit is used fortransmitting packets of both 2D video and depth map. One acknowledgementreceiving unit is employed to receive ACK or NACK signals for the sentpackets. One re-transmission determination unit is utilized to generateseparate re-transmission requests for 2D video and depth information;and one re-transmission unit is used to re-transmit the required packetupon receiving the re-transmission request.

In one embodiment, the normal transmission of the 2D video and depth mapare performed separately. There may be some offset between thetransmission start time for a 2D packet and that for a correspondingdepth packet. In one implementation, the start time of 2D packets may beearlier than that of depth packets so that the 2D packet transmissionstatus can be used to determine the depth packet's re-transmission. Notethat a large offset may lead to extra delay of the 3D content playbackbecause of the delayed depth map transmission. Thus, when selecting theoffset, there is a tradeoff between the3D content playback delay and there-transmission efficiency. In one embodiment, the offset could beselected to be close to the average round-trip time of a packet plussome adjustment time. Thus when a depth packet is transmitted, it isvery likely that the acknowledgement signal of its corresponding 2Dpacket has been received, which may facilitate the transmission of thedepth packet.

The re-transmission determination unit generates the re-transmissionrequest according to a re-transmission probability. The re-transmissionprobability for a 2D packet is determined based on the transmissionstatus of that 2D packet and time constraints, while the re-transmissionprobability for a depth packet is calculated based on the transmissionstatus and time constraints of that depth packet and of thecorresponding 2D packet. Generally speaking, the re-transmission of apacket would occur when there is a negative feedback about the packet,such as a NACK is received or a timeout of receiving the acknowledgementabout the packet has been observed. For a 2D packet, the probability ofthe re-transmission P_(2D) is correlated with the probability of theretransmitted packet arriving at the receiver successfully and withinthe time constraint, i.e. received before the deadline for playback.Thus, factors such as channel conditions, the remaining time before thedeadline may be taken into account. For a depth packet, except for thefactors that are considered for the calculation of P_(2D), thetransmission status of the corresponding 2D packet is also used as aninput for the determination of the probability of retransmitting a depthpacket, P_(d). If the corresponding 2D packet has a positive feedback(e.g. an ACK is received), then the re-transmission probability would behigh and vice versa. In other words, the probability to re-transmit thedepth packet P_(d) is related to and increases/decreases together withthe probability that the corresponding 2D packet is receivedsuccessfully.

Further, different negative feedbacks can also have different impact onthe determination of P_(d). For example, as can be seen in FIG. 3, whenthe NACK signal is received, there is higher chance for theretransmitted packet to arrive at the receiver than that when a timeoutis observed since there is more time left before the deadline forplayback. In this case, a higher P_(d) is assigned when the NACK signalis received, than when a timeout is observed.

FIG. 2 illustrates the process of controlling errors in the 3D videotransmission. Transmitting 2D packets and transmitting depth packetsshare the same major steps as shown in FIG. 2. In step 210, a 2D (ordepth) packet is transmitted. A determination is made at step 220 as towhether a timeout has occurred. If yes, the process moves on to step 240for calculation of the re-transmission probability; otherwise, step 230checks if an acknowledgement for the sent 2D (or depth) packet has beenreceived. If no acknowledgement has been received, the process keepschecking for timeout in step 220. If an ACK signal is received, theprocess moves back to step 210 for the normal transmission of a newpacket. If a NACK signal is received, the process moves onto step 240 tocalculate the re-transmission probability for the 2D (or depth) packet;and step 250 retransmits the packet with the calculated re-transmissionprobability.

In ARQ schemes, one of the important parameters is the number ofre-transmissions, which specifies the maximum number of re-transmissionattempts taken for a single packet delivery. If the number ofre-transmissions is reached, the packet is discarded. In most ofwireless network technologies, the number of re-transmissions is fixedto a default value, computed to provide to a certain radio linkbit-error-rate (BER) improvement tuned to an average case, e.g. tocompensate typical levels of signal fading and interference forpredefined packet sizes. It is clear that the cooperative ARQ of thepresent invention achieves an adaptive and a non-fixed number ofre-transmissions. The number of re-transmissions of a packet isdetermined by the re-transmission probability, which is furtherdetermined by the channel condition, the remaining time forre-transmission, and the transmission status of the corresponding 2Dpacket (for depth packet) etc. In situations when there is enough timeand high chance for the re-transmission to succeed, multiplere-transmissions may occur, while in situations when the chances aresmall, only one or no re-transmission may be proper. In this way, thenetwork connect is utilized more efficiently than the fixed number ofre-transmission.

In the following, one embodiment of the present invention is described.Again, 2D-plus-Depth is used as an example 3D format, which comprises a2D video and a depth map sequence. The same principle applies to otherformats that have been mentioned before.

Notations

T_deadline denotes a deadline for each packet to arrive at the receiverin order to achieve continuous playback of the transmitted video orother time-sensitive content.

T_out denotes timeout, which specifies periods of time being allowed toelapse before an acknowledgment is to be received.

T_curr denotes the current time.

T_trans denotes the time when a packet is being transmitted.

T_ack(T_nack) denotes the arrival time of an ACK(NACK) signal at thesender.

RTT denotes the round-trip time, which is a period of time for a sentsignal to arrive at the receiver plus the time for an acknowledgment ofthat signal to be received at the sender. RTT can be calculated in manyways. In one embodiment, the RTT for every signal which hasacknowledgement is recorded. The overall RTT is then updated as followsupon the arrival of a new acknowledgement of a signal.RTT=α×RTT_curr+(1−α)×RTT_prevwhere RTT_curr is the RTT of the latest acknowledged signal; RTT_prev isthe overall average RTT before the receipt of the latestacknowledgement; and α is a parameter that adjusts the weight that isput on the latest RTT when updating the overall RTT. An example value ofα is 0.1.ARQ for 2D data

The normal transmission unit transmits packets of the 2D video one byone. The acknowledgement receiving unit receives the feedback signals,such as ACK or NACK, from the receiver. The re-transmissiondetermination unit determines whether re-transmission is needed or not.If needed, such as when a negative feedback NACK is received or nofeedback is received before the timeout occurs, it makes a request tothe re-transmission unit. The re-transmission probability is determinedas follows and summarized in Table I:

-   -   1) If a NACK signal of a 2D packet is received, and        T_deadline−T_curr≧RTT/2+Δ (see FIG. 3), a re-transmission        request is sent to the re-transmission unit. In other words,        when a negative feedback of the 2D packet is received, the        re-transmission determination unit will calculate the remaining        time before the deadline, i.e. T_deadline−T_curr. If the        remaining time is long enough to transmit a packet, then a        re-transmission request is sent to the re-transmission unit with        probability p₀. The parameter Δ can take a value such as        σ_(RTT)/2, wherein σ_(RTT) is the standard deviation of the RTT.    -   2) If a NACK signal of a 2D packet is received, and        T_deadline−T_curr<RTT/2+Δ, then a re-transmission request is        sent to the re-transmission unit with a probability p₁ and        p₁<p₀.    -   3) If a timeout of a 2D packet occurs (i.e.        T_curr=T_trans+T_out), and T_deadline−T_curr≧RTT/2+Δ, then a        re-transmission request is sent to the re-transmission unit with        probability p₀.    -   4) If timeout of a 2D packet occurs(i.e. T_curr=T_trans+T_out),        and T_deadline−T_curr<RTT/2+Δ (see FIG. 3), then a        re-transmission request is sent to the re-transmission unit with        a probability p₁ and p₁<p₀.        The re-transmission unit transmits the required packet when        receiving the request.

TABLE 1 2D packet re-transmission probability (p₁ < p₀, in oneembodiment p₀ = 1) Conditions Results 2D NACK 2D timeout T_deadline −T_curr ≧ Re-transmission received occurs RTT/2 + Δ probability Yes — Yesp₀ Yes — No p₁ — Yes Yes p₀ — Yes No p₁ARQ for Depth Data

The re-transmission determination unit for depth data is slightlydifferent from that for 2D data. In comparison with ARQ for 2D data,this unit utilizes additional information on the corresponding 2D packettransmission when making re-transmission decision. The re-transmissionis determined as follows and summarized in Table II. The followingapplies when a NACK signal of a depth packet is received or a timeout ofa depth packet occurs (i.e. T_curr=T_trans+T_out).

-   -   1) If the ACK signal of the corresponding 2D packet has been        received, then send a re-transmission request to the        re-transmission unit with probability q₀ when        T_deadline−T_curr≧RTT/2+Δ or equivalently,        T_curr≦T_deadline−RTT/2−Δ, and send a re-transmission request to        the re-transmission unit with probability q₁ when        T_deadline−T_nack<RTT/2+Δ. In one embodiment, q₁<q₀.Similar to        the case for 2D video packet, the parameter Δ can take a value        such as σ_(RTT)/2, wherein σ_(RTT) is the standard deviation of        the RTT. Note that here and in the following scenarios, T_curr        is treated the same as T_nack since the decision is made upon        NACK signal is received, that is T_curr≈T_nack.    -   2) If no ACK/NACK signal of the corresponding 2D packet has been        received and timeout of the corresponding 2D packet has not        occurred, then send a re-transmission request to the        re-transmission unit with probability q₂ when        T_deadline−T_nack≧RTT/2+Δ, or equivalently        T_curr≦T_deadline−RTT/2−Δ, and send a re-transmission request to        the re-transmission unit with probability q₃ when        T_deadline−T_nack<RTT/2+Δ. In one embodiment, q₃<q₂.    -   3) If, for the corresponding 2D packet, a NACK signal is        received or a timeout has occurred, and T_nack (or        T_timeout)≦T_deadline−RTT/2−Δ (for the corresponding 2D packet),        then send a re-transmission request to the re-transmission unit        with a probability q₄ when T_deadline−T_nack≧RTT/2+Δ (for the        depth packet), or equivalently T_curr≦T_deadline−RTT/2−Δ, and        send a re-transmission request to the re-transmission unit with        a probability q₅ when T_deadline−T_nack<RTT/2+Δ. In one        embodiment, q₅<q₄. T_timeout denotes the time of the recent        timeout of the corresponding 2D video packet that occurred.    -   4) If, for the corresponding 2D packet, a NACK signal is        received or a timeout has occurred, and T_nack (or        T_timeout)>T_deadline−RTT/2−Δ (for the corresponding 2D packet),        then send a re-transmission request to the re-transmission unit        with a probability q₆ when T_deadline−T_nack≧RTT/2+α (for the        depth packet), or equivalently T_curr≦T_deadline−RTT/2−Δ, and        send a re-transmission request to the re-transmission unit with        a probability q₇ when T_deadline−T_nack<RTT/2+Δ. In one        embodiment, q₇<q₆. T_timeout denotes the time of the recent        timeout of the corresponding 2D video packet that occurred.    -   5) In the above scenarios 1)-4), we assume T_curr<T_deadline.        When T_curr>T_deadline, no re-transmission request is sent.        In one implementation, q₀, q₂, q₄ are set to be greater than        0.7, while q₁, q₃ q₅ are less than 0.3. q₇ could be set to 0. In        another embodiment, q₀>q₂, q₄, q₆.

TABLE 2 Depth packet re-transmission probability Conditions DepthPacketCorresponding 2DPacket Depth Corresponding T_nack(or NACK T_curr ≦Corresponding 2D NACK T_timeout) ≦ received T_deadline − 2D received orT_deadline − RTT/2− Results or timeout RTT/2− Δ T_curr ≦ ACK timeout hasΔ (Corresponding Re-transmission occurs (Depth) T_deadline receivedoccurred 2D) probability Yes Yes — Yes — — q₀ Yes No Yes Yes — — q₁ YesYes No No — q₂ Yes No Yes No No — q₃ Yes Yes — No Yes Yes q₄ Yes No YesNo Yes Yes q₅ Yes Yes — No Yes No q₆ Yes No Yes No Yes No q₇ — — No — —— 0

In a different embodiment, the above probability for re-transmitting 2Dand depth packet, i.e. P_(2D) and P_(d), may be calculated as a functionof the remaining time for re-transmission (T_deadline−T_curr) and thethreshold RTT/2+Δ. For example, the function may allow the P_(2D)decrease as (T_deadline−T_curr)−(RTT/2+Δ) decreases. Thus, theprobabilities p₀, p₁, q₀˜q₇would be different for different packets.

Although the present invention is described in the context of the 3Dvideo transmission, the same principles apply to any data transmissionwherein the data comprises two or more interrelated components/files andthe packets in each of the components/files have one to onecorrespondence.

It is further to be understood that the present invention may beimplemented in various forms of hardware, software, firmware, specialpurpose processors, or a combination thereof. Preferably, the presentinvention is implemented as a combination of hardware and software.Moreover, the software is preferably implemented as an applicationprogram tangibly embodied on a program storage device. The applicationprogram may be uploaded to, and executed by, a machine comprising anysuitable architecture. Preferably, the machine is implemented on acomputer platform having hardware such as one or more central processingunits (CPU), a random access memory (RAM), and input/output (I/O)interface(s). The computer platform also includes an operating systemand microinstruction code. The various processes and functions describedherein may either be part of the microinstruction code or part of theapplication program (or a combination thereof), which is executed viathe operating system. In addition, various other peripheral devices maybe connected to the computer platform such as an additional data storagedevice and a printing device.

Although preferred embodiments of the present invention have beendescribed in detail herein, it is to be understood that this inventionis not limited to these embodiments, and that other modifications andvariations may be effected by one skilled in the art without departingfrom the scope of the invention as defined by the appended claims.

The invention claimed is:
 1. A method for controlling errors during datatransmission, wherein said data comprise two interrelated components,said method comprising: transmitting a first packet of a first componentand a second packet of a second component of said data separately;determining a re-transmission probability for said second packet basedon a transmission status of said first packet; and re-transmitting saidsecond packet according to said re-transmission probability for saidsecond packet, wherein said data is a 3D video sequence and wherein said3D video sequence comprises a 2D video component and a depth mapcomponent.
 2. The method of claim 1, further comprising: determining are-transmission probability for said first packet based on thetransmission status of said first packet and a time constraint of saidfirst packet; and re-transmitting -said first packet with saidre-transmission probability for said first packet.
 3. The method ofclaim 1, wherein said data is a 3D video sequence and wherein said 3Dvideo sequence comprises a left view component and a right viewcomponent.
 4. The method of claim 1, wherein said re-transmissionprobability for said second packet is further determined based on a timeconstraint of said second packet.
 5. The method of claim 4, wherein saidtime constraint comprises receiving said second packet before adeadline.
 6. The method of claim 1, wherein said transmission status ofsaid first packet comprises at least one of an acknowledgement (ACK)signal of said first packet, a negative acknowledgement (NACK) signal ofsaid first packet, and a timeout for receiving acknowledgement for saidfirst packet.
 7. The method of claim 6, wherein said probability ofre-transmission for said second packet is higher when said transmissionstatus of said first packet is an acknowledgement (ACK) signal, thanthat when said transmission status of said first packet is one of anegative acknowledgement (NACK) signal and a timeout.
 8. An apparatusfor controlling errors during data transmission, wherein said datacomprise two interrelated components, said method comprising: a normaltransmission unit for transmitting a first packet of a first componentand a second packet of a second component of said data separately; are-transmission determination unit for generating a re-transmissionrequest for said second packet based on a transmission status of saidfirst packet; and a re-transmission unit for re-transmitting said secondpacket upon receiving said re-transmission request, wherein said data isa 3D video sequence and wherein said 3D video sequence comprises a 2Dvideo component and a depth map component.
 9. The apparatus of claim 8,wherein said re-transmission determination unit generates saidre-transmission request for said second packet according to a firstprobability, and wherein said first probability is calculated based onsaid transmission status of said first packet.
 10. The apparatus ofclaim 8, wherein said re-transmission determination unit generates are-transmission request for said first packet based on a transmissionstatus and a time constraint of said first packet; and saidre-transmitting unit re-transmits said first packet upon receiving saidre-transmission request for said first packet.
 11. The apparatus ofclaim 10, wherein said re-transmission determination unit generates saidre-transmission request for said first packet according to a secondprobability, and wherein said second probability is calculated based onsaid transmission status and said time constraint of said first packet.12. The apparatus of claim 8, wherein said data is a 3D video sequenceand wherein said 3D video sequence comprises a left view component and aright view component.
 13. The apparatus of claim 9, wherein said firstprobability is further determined based on a time constraint of saidsecond packet.
 14. The apparatus of claim 13, wherein said timeconstraint comprises receiving said second packet before a deadline. 15.The apparatus of claim 9, wherein said transmission status of said firstpacket comprises at least one of an acknowledgement (ACK.) signal ofsaid first packet, a negative acknowledgement (NACK) signal of saidfirst packet, and a timeout for receiving acknowledgement for said firstpacket.
 16. The apparatus of claim 15, wherein said first probabilityfor said second packet is higher when said transmission status of saidfirst packet is an acknowledgement (ACK) signal, than that when saidtransmission status of said first packet is one of a negativeacknowledgement (NACK) signal and a timeout.